Announcements. Chapter 3

Transcription

1 Announcements Project 2 has been posted Due Feb 1st at 10:00pm Work ALONE! Help hours Monday Thursday, 7 10pm LWSN B146 Quiz solutions will be on newsgroup Sign up for newsgroup! We expect you to be reading book. These slides should be a review 1

2 Flow of Control 2

3 Outline Branching Statements Java Loop Statements Programming with Loops The Type boolean (optional) Graphics Supplement 3

4 Compound Statements To include multiple statements in a branch, enclose the statements in braces. if (count < 3) { total = 0; count = 0; } 4

5 Using ==, cont. is not appropriate for determining if two objects have the same value. if (s1 == s2), where s1 and s2 refer to strings, determines only if s1 and s2 refer the a common memory location. If s1 and s2 refer to strings with identical sequences of characters, but stored in different memory locations, (s1 == s2) is false. == 5

6 Multibranch if else Statements, cont. class Grader 6

7 The switch Statement The switch statement is a mutltiway branch that makes a decision based on an integral (integer or character) expression. The switch statement begins with the keyword switch followed by an integral expression in parentheses called the controlling expression. 7

8 The switch Statement, cont. A list of cases follows, enclosed in braces. Each case consists of the keyword case followed by a constant called the case label a colon a list of statements. The list of cases is searched in order for a case label matching the controlling expression. 8

9 The switch Statement, cont. The action associated with a matching case label is executed. If no match is found, the case labeled default is executed. The default case is optional, but recommended, even if it simply prints a message. Repeated case labels are not allowed. 9

10 The switch Statement, cont. class MultipleBirths 10

11 The switch Statement, cont. The action for each case typically ends with the word break. The optional break statement prevents the consideration of other cases. The controlling expression can be anything that evaluates to an integral type. 11

12 The for Statement A for statement executes the body of a loop a fixed number of times. example for (count = 1; count < 5; count++) System.out.println(count); System.out.println( Done ); 12

13 Choosing a Loop Statement If you know how many times the loop will be iterated, use a for loop. If you don t know how many times the loop will be iterated, but it could be zero, use a while loop it will be at least once, use a do while loop. Generally, a while loop is a safe choice. 13

14 The break Statement in Loops A break statement can be used to end a loop immediately. The break statement ends only the innermost loop or switch statement that contains the break statement. break statements make loops more difficult to understand. Use break statements sparingly (if ever). 14

15 The break Statement in Loops, cont. class BreakDemo 15

16 The exit Method Sometimes a situation arises that makes continuing the program pointless. A program can be terminated normally by System.exit(0). example if (numberofwinners == 0) { System.out.println( cannot divide by 0 ); System.exit(0); } 16

17 Programming with Loops: Outline The Loop Body Initializing Statements Ending a Loop Loop Bugs Tracing Variables 17

18 The Loop Body To design the loop body, write out the actions the code must accomplish. Then look for a repeated pattern. The pattern need not start with the first action. The repeated pattern will form the body of the loop. Some actions may need to be done after the pattern stops repeating. 18

19 Initializing Statements Some variables need to have a value before the loop begins. Sometimes this is determined by what is supposed to happen after one loop iteration. Often variables have an initial value of zero or one, but not always. Other variables get values only while the loop is iterating. 19

20 Ending a Loop If the number of iterations is known before the loop starts, the loop is called a count controlled loop. use a for loop. Asking the user before each iteration if it is time to end the loop is called the ask before iterating technique. appropriate for a small number of iterations Use a while loop or a do while loop. 20

21 Ending a Loop, cont. For large input lists, a sentinel value can be used to signal the end of the list. The sentinel value must be different from all the other possible inputs. A negative number following a long list of nonnegative exam scores could be suitable

22 Ending a Loop, cont. example reading a list of scores followed by a sentinel value int next = keyboard.nextint(); while (next >= 0) { Process_The_Score next = keyboard.nextint(); } 22

23 Ending a Loop, cont. class ExamAverager 23

24 Nested Loops The body of a loop can contain any kind of statements, including another loop. In the previous example the average score was computed using a while loop. This while loop was placed inside a do while loop so the process could be repeated for other sets of exam scores. 24

25 Declaring Variables Outside Loop Bodies The declaration of variables inside a loop body is repeated with each execution of the loop body. This can be inefficient, depending on the compiler. It the declaration of variables can be moved outside the loop body, generally it is appropriate to do so. 25

26 Loop Bugs common loop bugs unintended infinite loops off by one errors testing equality of floating point numbers subtle infinite loops The loop may terminate for some input values, but not for others. For example, you can t get out of debt when the monthly penalty exceeds the monthly payment. 26

27 Subtle Infinite Loops Verify that the monthly payment exceeds the penalty, for example, before entering a loop to determine the number of payments needed to get out of debt. if (payment <= penalty) System.out.println( payment is too small ); else {... 27

28 Off by One Errors The loop body is repeated one too many times or one too few times. examples < is used when <= should be used or <= is used when < should be used using the index of the last character of a string instead of the length of the string (or vice versa) easy to overlook 28

29 Testing Equality of Floating point Numbers works satisfactorily for integers and characters. == is not reliable for floating point numbers (which are approximate quantities). Use <= or >= rather than == or!=. == 29

30 Tracing Variables Tracing variables means watching the variables change while the program is running. Simply insert temporary output statements in your program to print of the values of variables of interest or, learn to use the debugging facility that may be provided by your system. 30

31 Tracing Variables, cont. 31

32 The Type boolean Boolean Expressions and Variables Truth Tables and Precedence Rules Input and Output of Boolean Values 32

33 The Type boolean, cont. The type boolean is a primitive type with only two values: true and false. Boolean variables can make programs more readable. if (systemsareok) instead of if((temperature <= 100) && (thrust >= 12000) && (cabinpressure > 30) && ) 33

34 Boolean Expressions and Variables Variables, constants, and expressions of type boolean all evaluate to either true or false. A boolean variable can be given the value of a boolean expression by using an assignment operator. boolean ispositive = (number > 0);... if (ispositive)... 34

35 Naming Boolean Variables Choose names such as ispositive or systemsareok. Avoid names such as numbersign or systemstatus. 35

36 Truth Tables 36

37 Precedence Rules Parentheses should be used to indicate the order of operations. When parentheses are omitted, the order of operation is determined by precedence rules. 37

38 Precedence Rules, cont. Operations with higher precedence are performed before operations with lower precedence. Operations with equal precedence are done left to right (except for unary operations which are done right to left). 38

39 Precedence Rules, cont. 39

40 Precedence Rules, cont. In what order are the operations performed? score < min/2 10 score > 90 score < (min/2) 10 score > 90 score < ((min/2) 10) score > 90 (score < ((min/2) 10)) score > 90 (score < ((min/2) 10)) (score > 90) 40

41 Short circuit Evaluation Sometimes only part of a boolean expression needs to be evaluated to determine the value of the entire expression. If the first operand associated with an is true, the expression is true. If the first operand associated with an && is false, the expression is false. This is called short circuit or lazy evaluation. 41

42 Short circuit Evaluation, cont. Short circuit evaluation is not only efficient, sometimes it is essential! A run time error can result, for example, from an attempt to divide by zero. if ((number!= 0) && (sum/number > 5)) Complete evaluation can be achieved by substituting & for && or for. 42

43 Input and Output of Boolean Values example boolean boo = false; System.out.println(boo); System.out.print( Enter a boolean value: ); Scanner keyboard = new Scanner (System.in); boo = keyboard.nextboolean(); System.out.println(boo); 43

44 Input and Output of Boolean Values, cont. dialog false Enter a boolean value: true true 44

45 Using a Boolean Variable to End a Loop example boolean numberslefttoread = true while (numberslefttoread) { next = keyboard.nextint() if (next < 0) numberslefttoread = false; else Process_Next_Number } 45

46 Using a Boolean Variable to End a Loop, cont class BooleanDemo 46

47 (optional) Graphics Supplement: Outline Specifying a Drawing Color The drawstring Method A JOptionPane Yes/No Window 47

48 Specifying a Drawing Color When drawing a shape inside an applet s paint method, think of the drawing being done with a pen that can change colors. The method setcolor changes the color of the pen. canvas.setcolor(color.yellow); Drawings done later appear on top of drawings done earlier. 48

49 Specifying a Drawing Color, cont. 49

50 Specifying a Drawing Color, cont. 50

51 Programming Example class MultipleFaces 51

52 Programming Example, cont. class MultipleFaces, contd. 52

53 Programming Example, cont. 53

54 The drawstring Method similar to other drawing methods, but used to draw text canvas.drawstring( Hello,10,20); syntax Graphics_Object.drawString(String, X, Y); 54

55 A JOptionPane Yes/No Window used to present the user with a yes/no question The window contains the question text two buttons labeled Yes and No. 55

56 A JOptionPane Yes/No Window, cont. example int answer = JOptionPane.showConfirmDialog(null, End program?, Want to end?, JOptionPane.YES_NO_OPTION); if (answer == JOptionPane.YES_OPTION) System.exit(0); else System.out.println( once more ); 56

57 A JOptionPane Yes/No Window, cont. 57

58 A JOptionPane Yes/No Window, cont. returns an int value named either YES_OPTION or NO_OPTION, but you do not need to think of them as ints. The second argument ( End program? in our example) appears in the window. The third argument ( Want to end? in our example) is displayed as the title of the window. JOptionPane.showConfirmDialog 58

59 A JOptionPane Yes/No Window, cont. The last argument (JOptionPane.YES_NO_OPTION in our example). requests a window with yes and no buttons. The first argument (null in our example) affects the placement of the window on the screen. Simply use null for now. 59

60 Summary You have learned about Java branching statements. You have learned about loops. You have learned about the type boolean. (optional) You have learned to use color and the JOptionPane yes/no window. 60